An explosion that killed seven Camp Lejeune Marines -- including Lance Cpl. Roger W. Muchnick Jr., a 2008 Staples High School graduate -- during a nighttime training exercise last year was caused by human error and insufficient training, according to the results of a military investigation.

Lt. Adam Flores, a spokesman for the Lejeune-based 2nd Marine Division, said Wednesday the investigation found the deadly explosion on March 18, 2013, was triggered when one of the Marines dropped a second round into an already loaded mortar tube during a live-fire exercise.

Two officers and a noncommissioned officer were relieved of command following the explosion at Hawthorne Army Depot in Nevada.

"Actually it contained a lot less than what I already knew," said Coakley, who lives in Florida. "It's not as detailed as the information the families received." She said she also got "very accurate" accounts from "others on the ground" at the time of the explosion, when they attended her son's funeral.

"They perished doing an honorable job," she said.

The 23-year-old Muchnick -- known as "R.J." to family and friends -- was one of the Marines killed in the blast from the 60-mm mortar. The other fatalities in the unit -- Alpha Co., 1st Battalion, 9th Marines, based at Camp Lejeune, N.C. -- ranged in age from 19 to 26. Seven more Marines and a Navy sailor were also wounded in the accident.

"It was human error," said Coakley. "I can't speak for the other families, but I don't hold anyone accountable."

She said the Marines were training for combat and the incident "underscores the danger of working with bombs and weaponry."

The investigation determined that the mortar functioned properly and that the weapon system is safe when used as designed by Marines with the requisite training. Marine officials said last year that the explosion was the result of "human error," but did not provide details of what caused the blast.

Flores said Wednesday that the Marines were firing the 60-mm mortar in "manual mode," meaning someone had to pull a trigger to set off the round. The weapon can also be set up in a fixed position and fired by simply dropping a round down the tube.

"There was a round that was already in and they had begun to feed the second round in before the first mortar (round) had been fired," Flores said of the ignition.

The investigation's findings were first reported Tuesday by The Marine Corps Times, which obtained a copy of the 19-page report and hundreds of pages of supporting documents through a Freedom of Information Act request.

The Marine investigation concluded that four factors contributed to the tragedy: inadequate training and preparation for the complexity of the exercise; improper mortar gunnery commands and firing procedures; a "perceived sense of urgency and resultant haste" during the exercise; and a systemic lack of supervision.

Coakley said injuries and deaths periodically take place during training, but the incident involving her son was highlighted because of the number of Marines killed and injured.

"They got a lot of press because there were so many," she said. "It happens a lot, " she added. "They have to train that way."

A family friend, Eric Brand, was at Coakley's home Thursday. A Marine for 12 years -- in both active service and as a reservist -- Brand was also injured during a training exercise. "It's an inherently dangerous job," he said. "They have high standards in the Marines."

Muchnick, who joined the Marines in June 2010, had served in the Middle East during Operation Enduring Freedom in 2011.

While at Staples, Muchnick played both lacrosse and football. In a 2008 profile of Muchnick in the Westport News, he talked about the benefits he found in playing both sports: "Playing football and lacrosse year-round makes you into a better athlete."

He was well-liked, both off and on Staples' athletic fields.

"He was the kind of kid, who if he ever decided to become a salesman, there'd be no stopping him," Staples Principal John Dodig said of Muchnick at the time of his death. "He had an electric personality."

Despite his competitive nature, Muchnick had several hundred friends listed on his Facebook page. And he knew every one of them, his mother recalled last year after learning of his death.

"R.J. was one of the most social people you ever met. He has friends who he's known since he was 2 years old. He has friends from preschool and all the way up through Staples and beyond," Coakley said in an interview. "He's never lost a friend. Not his whole life."